I. Field
The following description relates generally to wireless communications, and more particularly to schemes for generating equalizer coefficients in a MIMO system.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as for example voice, media, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP LTE systems, 3GPP2 UMB systems, and orthogonal frequency division multiple access (OFDMA) systems.
An orthogonal frequency division multiple access (OFDMA) system utilizes orthogonal frequency division multiplexing (OFDM). OFDM is a multi-carrier modulation technique that partitions overall system bandwidth into multiple (N) orthogonal frequency subcarriers. These subcarriers may also be called tones, bins, and frequency channels. Each subcarrier is associated with a respective sub carrier that may be modulated with data. Up to N modulation symbols may be sent on the N total subcarriers in each OFDM symbol period. These modulation symbols are converted to the time-domain with an N-point inverse fast Fourier transform (IFFT) to generate a transformed symbol that contains N time-domain chips or samples.
In a frequency hopping communication system, data is transmitted on different frequency subcarriers in different time intervals, which may be referred to as “hop periods”. These frequency subcarriers may be provided by orthogonal frequency division multiplexing, other multi-carrier modulation techniques, or some other constructs. With frequency hopping, the data transmission hops from subcarrier to subcarrier in a pseudo-random manner. This hopping provides frequency diversity and allows data transmission to better withstand deleterious path effects such as narrow-band interference, jamming, fading, and so on.
An OFDMA system can support multiple mobile stations concurrently. For a frequency hopping OFDMA system, data transmission for a given mobile station may be sent on a “traffic” channel that is associated with a specific frequency hopping (FH) sequence. This FH sequence indicates the specific subcarrier to use for data transmission in each hop period. Multiple data transmissions for multiple mobile stations may be sent concurrently on multiple traffic channels associated with different FH sequences. These FH sequences may be defined to be orthogonal to one another so that only one traffic channel, and thus only one data transmission, uses each subcarrier in each hop period. By using orthogonal FH sequences, multiple data transmissions generally do not interfere with one another while enjoying benefits of frequency diversity.
Generally, a wireless multiple-access communication system can concurrently support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to a communication link from base stations to terminals, and the reverse link (or uplink) refers to a communication link from terminals to base stations. This communication link may be established via a single-input-single-output (SISO), multiple-input-single-output (MISO), single-input-multiple-output (SIMO) or a multiple-input-multiple-output (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that a reciprocity principle allows estimation of the forward link channel from the reverse link channel. This enables an access point to extract and transmit beamforming gain on the forward link when multiple antennas are available at the access point.
A conventional MIMO wireless system has 2 modes of operation—single code word (SCW) and multiple-code word (MCW). In MCW mode, for each tone the transmitter can encode data transmitted on each spatial layer independently, possibly with different rates. The receiver employs a successive interference cancellation (SIC) algorithm and can employ a low complexity linear receiver such as minimum mean-squared error (MMSE) receiver or a zero-forcing (ZF) receiver, or non-linear receivers, for each tone.
MIMO design requires use of MMSE equalizers at the receiver to separate MIMO spatial multiplexing streams. The MMSE equalization consists of (a) Equalizer coefficient computation (b) SINR and bias computation for each data symbol; and (c) Demodulation of data symbols. For a conventional receiver, equalizer coefficients are generated for each SIC layer by separately computing individual matrix inverses. This can increase computational complexity of an SIC receiver. Thus, there exists a need in the art for a system and/or methodology that automatically generates SIC equalizer coefficients for multiple layers, with minimal overhead in complexity.